The invention relates to a joint base for a hip joint endoprosthesis, comprising a base element of elongated shape and a base insert (inlay) which has at the front side a semi-spherical hollow for accommodating a joint ball.
Endoprostheses as hip joint replacement have been known for a long time. Reference is made to DE-U 72 40 856 or DE-A 26 11 985. In the DE-A 23 01 810 is described a joint base consisting of an outer and an inner cup, and the inner cup is detachably accommodated in the outer cup and itself accommodates a joint ball. In the EP-B 0 303 006 is described a revision base, which is thus suited in particular as replacement for an implanted joint base with detailed base roof of the acetabulum. This joint base includes a base element which has at the front side a semi-spherical hollow for accommodating a joint base, and the axis of the hollow lies eccentrically of the base element and the joint base is cross-sectionally perpendicular to the eccentrically lying axis of the hollow of elongated oval shape. Thus, the known joint base provides a joint element of elongated shape which is symmetrical both in the longitudinal direction and in the transverse direction thereto, i.e. virtually forming an oval. The semi-spherical hollow preferably lies in an area in which the anatomic hollow is positioned in the acetabulum. The idea of this design is that the joint base closely resembles the anatomy as it develops by a loosened prosthetic joint base with the base roof wearing down. Accordingly, insertion of the joint base requires only minimal removal or replacement of bone substance for insertion of the joint base.
However, a disadvantage of the prior art is the symmetrical design of the joint base, as a result of which usually only so-called two-point tensioning of it inside the acetabulum can be achieved. Around the connecting line between these two points, tilting moments become effective which after an extended period of implantation can result in loosening of the base. Then, the joint base has to be again replaced.
It is an object of the present invention to produce a joint base of the aforementioned type which permits statically determined tensioning within the acetabulum, so that no tilting moments can become active which result in loosening of the base. It is a particular aim to achieve defined multi-point tensioning.
An object of the invention involves a joint base for a hip joint endoprosthesis. The joint base includes an elongate base element and a base insert. The joint base has a longitudinal axis and an outline which, in a front view, is of asymmetrical design at least relative to the longitudinal axis. The base insert has at a front side a semi-spherical hollow for accommodating a joint ball. In this respect, reference is made to an embodiment, according to which the joint base has, as seen in a frontal view, amongst other things a kidney- or beanshaped outline. This outline results in three-point tensioning at the convex extreme points. This tensioning is statically determined and as such accordingly stable.
Alternatively, the outline of the joint base could be of trapezoidal, triangular, halfmoon, heart, arrow or semi-circular or similar asymmetrical shape. It only has to be ensured that statically determined multi-point tensioning is achieved in order to obtain a stable implantation.
Furthermore, an aspect of the invention involves an embodiment, according to which the base insert is fixed within a corresponding cutout in the joint base, i.e. in different angular positions relative to the center axis of the aforementioned cutout, and the cutout is preferably of concentrical design in the base element. For an eccentrically arranged semi-spherical hollow in the base insert, this hollow can then be placed into a desired position relative to the base element. Thus, both the cranio/caudal (i.e., top/bottom) position of the semi-spherical hollow as well as its medio/lateral position can be varied. This is of advantage when the base cannot be implanted in an ideal position due to bone defects.
Furthermore, an aspect of the invention involves an embodiment, according to which the front of the base element is in the cranial area covered in a forward direction whilst it is retracted toward the inside or rear in the caudal area. By covering in the cranial area, a luxation of the joint ball in the steep position of the base is prevented. In this case, retraction in the caudal area also permits sufficient adduction of the leg.
It is also of advantage if the bottom of the base element comprises one or more passages which is/are closable by a sealing cover, in particular a displaceably or rotary mounted sealing cover. This makes it possible, after insertion of the base element into the pelvis of the patient, to control the setting depth and spongiosa between bone and bottom of the base element. In order to prevent contact between the base insert or inlay, which is preferably made of plastic, and the bone, the aforementioned passage in the bottom of the base element is sealed with a sealing cover, i.e. preferably a rotary mounted sealing cover. An alternative solution provides shaping the rear or outside of the base insert of the same material or a similar material as the base element and in such a manner that on insertion of the base insert into the base element the rear of the base insert closes one or more passages in the bottom of the base element like a sealing cover. This solution is very advantageous in particular for handling or manufacturing.
In order to additionally increase fixing of the base element, the latter may comprise at the outside a plurality of engagement webs with knifelike cutters spaced virtually evenly over the circumference, and the engagement webs extend approximately parallel to the center axis of the joint element.
Furthermore, the base element can for this purpose comprise holes in the cranial and/or caudal area for passing through bone screws, and the screw holes are preferably like a sand-glass or venturi-tube so that the screws can be screwed in without force at different angles. It is also possible to provide bungs for closing unused screw holes. The screw holes are preferably located between the aforementioned engagement webs of the base element.
Fixing the base insert in a corresponding cutout of the base element is advantageously achieved by a snap-on mechanism. It is feasible to provide for this purpose at the outside of the base insert a peripheral bead which engages a corresponding annular groove within the cutout of the base element.
The surface of the semi-spherical hollow configured in the base insert is preferably provided with a gliding layer, in particular a gliding layer of metal, ceramic or a friction resistant plastic material. The base insert can also be made in total of one of the aforementioned materials, in particular ceramics or a combination of these materials.
For the purpose of fixing the base insert within a corresponding cutout in the base element at a predetermined angle relative to the center axis of the aforementioned cutout, the base insert can have at the side of the edge at least one radially outward extending protrusion or alternatively at least one recess, which correspond(s) with at least one recess in the cutout of the base element or one protrusion in the cutout of the base element. Preferably a plurality of protrusions or recesses is arranged evenly spaced over the circumference.